The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which upon heating decomposes to produce liquid and gaseous hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil". A number of methods have been proposed for processing oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by reference. This patent describes in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing "retorted oil shale".
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of any oxygen containing retort inlet mixture into the retort as a gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to product heat and combustion gas. By the continued introduction of the retort inlet mixture containing oxygen into the retort, the combustion zone is advanced through the fragmented mass.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid products including gaseous and liquid hydrocarbon products and to a residue of solid carbonaceous material.
The liquid products and gaseous products are cooled by cooler particles in the fragmented mass in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas generated in the combustion zone, gaseous products produced in the retorting zone, gas from carbonate decomposition, and the gaseous portion of retort inlet mixture that does not take part in the combustion process is withdrawn from the bottom of the retort.
Parker in U.S. Pat. No. 3,454,958 describes a method for producing oil from oil shale in a nuclear chimney having a void at the top by igniting shale in the top of the chimney to establish a combustion zone. Parker teaches that the combustion zone may be established by burning a bed of charcoal soaked in a suitable fuel placed in the void. Although the method of Parker can be used in a retort having a void at the top for placement of charcoal such as a retort produced by a nuclear explosive, it is useless for a retort which is filled with a fragmented permeable mass containing oil shale and thus has little or no void at the top.
Establishment of a combustion zone in the retort can be effected according to the method described in U.S. Patent application Ser. No. 578,203 filed May 16, 1975, and now U.S. Pat. No. 4,027,917 and 3,952,801 issued Apr. 27, 1976 to Robert S. Burton III, and assigned to the assignee of this application. Both this patent application and patent are incorporated herein by this reference. The patent to Burton describes a technique for establishing a combustion zone in a retort by igniting the top of a fragmented permeable mass in the retort. According to this technique, a hole is bored to the top of the fragmented permeable mass and a burner is lowered through the bore hole to the oil shale to be ignited. A mixture of a combustible fuel such as LPG (liquified petroleum gas) and gas containing oxygen, such as air, is burned in the burner and the resultant flame is directed downwardly towards the fragmented permeable mass. The burning is conducted until a substantial portion of the oil shale has been heated above its ignition temperature so combustion of oil shale in the fragmented mass is self-sustaining. Then introduction of fuel is terminated, the burner is withdrawn from the retort through the hole, and oxygen supplying gas is introduced to the retort to advance the combustion zone through the retort.
It can be time consuming to establish a combustion zone in a retort. For example, a startup time as long as a week has been experienced with a retort in the south/southwest portion of the Piceance Creek structural basin in Colorado. Such a long startup time results in consumption of large quantities of LPG, an expensive, premium fuel.
An in situ oil shale retort can have a substantial lateral extent. For example, it can be square with a lateral dimension of 100 feet or more. With such a large retort, a large number of burners and bore holes to various portions of the top of the retort and large quantities of fuel such as LPG can be required for establishing a combustion zone in the retort. Preparation of a large number of bore holes and use of a large number of burners and large quantities of LPG can contribute significantly to the cost of producing hydrocarbon products from oil shale.
A method for establishing a combustion zone in a retort as described in U.S. Pat. No. 3,952,801 can result in establishment of a combustion zone which is skewed and/or warped if only a few burners are used for establishing a combustion zone. Use of more than a few burners to avoid a skewed or warped combustion zone can significantly increase the cost of establishing a combustion zone in a retort and producing shale oil. It is desirable to maintain a combustion zone which is flat and uniformly transverse to the direction of its advancement to maximize yield of hydrocarbon products from the oil shale in an in situ oil shale retort. If the combustion zone is skewed relative to its direction of advancement, there is more tendency for oxygen present in the combustion zone to migrate into the retorting zone, thereby oxidizing hydrocarbon products produced in the retorting zone and reducing hydrocarbon yield. In addition, with a skewed and/or warped combustion zone, excessive cracking of hydrocarbon products produced in the retorting zone can result.
Around each ignition point of situs in the fragmented permeable mass, a combustion zone is formed which tends to progress downwardly and laterally in the fragmented permeable mass. The combustion zone advances downwardly through the fragmented mass primarily by gas flowing through the retort and advances laterally and radially in the fragmented mass primarily by conduction and radiation. Since heat transfer by conduction and radiation through a fragmented mass of formation particles is much slower than heat transfer by convection, a substantial amount of unretorted oil shale can be left in the "corners" or side edges adjacent the walls of a retort. This can significantly reduce the yield of hydrocarbons obtained from the retort.
Thus, it is desirable to provide a low cost and fast method for establishing a combustion zone an in situ oil shale retort where the combustion zone is flat and uniformly transverse to its direction of advancement and extends laterally to the walls of the retort.